A heating device (also called an electric heater) functions based on the heating effects of electric current, i.e. heat produced while a current passes through a thermal resist. Generally, the thermal resist is made of conducting material with a high thermal resistivity and a high melting point. Metal, such as Ni—Cr, Fe—Cr—Al, Mn—Cu, W and the like, is often used as thermal resist in conventional heating devices. However, since oxidation accompanies the heating process, a metal thermal resist becomes thinner with time and finally breaks and damages the heating device. Nonmetal thermal resists such as ceramic also can be used in the heating device, however, nonmetal thermal resists have high cost since the manufacturing process is complicate and time-consuming. The above conventional heating devices merely produce 10 degrees Celsius (° C.) to 200° C. under about 110-220V of high voltages.
In the technical field, carbon nanotubes may be used as thermal resist, and said carbon nanotubes are the film composed of carbon nanotube. Conventional methods of manufacturing carbon nanotubes include arc discharge, laser evaporation, chemical vapor deposition (CVD), and the like. Arc discharge was the earliest developed method to synthesize carbon nanotubes. With arc discharge, the material source is gasified under the high temperature (about 4000K) and then deposed as nano material. However, the disadvantages of arc discharge include short nanotubes, low yield, high impurities, high cost, and difficult commercialization.
The principle of laser evaporation is similar to arc discharge, but a high energy laser is used to replace the arc. A graphite bar containing a catalyst is gasified instantaneously by the high energy laser and then cooled to form carbon nanotubes. The purity of carbon nanotubes from laser evaporation is better than that from arc discharge. However, it is difficult to control the process so that it is difficult to control the length, tube diameter, and arrangement of carbon nanotube. Further, the laser evaporation method cannot be applied in large scale manufacture because of the limited equipments.
In the CVD method, when a gaseous hydrocarbon passes by a substrate, the gaseous hydrocarbon is decomposed under 600-1200° C., which is a catalyst to form carbon nanotubes. CVD includes the tube furnace heating method and the microwave-plasma heating method. The carbon nanotube manufactured from CVD has high purity. The reaction temperature is lower, and the growing area can be designated. Conventional CVD uses the single-section or the double-section tube furnace to form carbon nanotubes. However, the conventional CVD method has disadvantages including low uniformity of carbon nanotubes, incomplete reaction of catalyst, slow growing of carbon nanotubes, and so on. Because CVD is the main method for large-scale production of carbon nanotube, an improved process is still needed.